1. Field of the Invention
The present invention relates to a method for assembling a heater-equipped air-fuel ratio sensor, which is preferably installed on an exhaust gas emission system of an internal combustion engine for automotive vehicles and others.
2. Related Art
In general, to improve the exhaust gas purification efficiency, an air-fuel ratio of exhaust gas is sensed by an air-fuel ratio sensor, such as an oxygen (O.sub.2) sensor. An air-fuel ratio sensor usually comprises a cylindrical or cup-shaped sensor element having an open end and an opposed closed end with an inside chamber. A stick-like heater is inserted in the inside chamber of this sensor element.
The heater is necessary to warm up the sensor element quickly to its active temperature during an engine start-up condition where the engine is operated at a low temperature.
More specifically, a portion sensing an air-fuel ratio of exhaust gas is a particular portion of the sensor element which is subjected to the exhaust gas (i.e. gas to be sensed), normally the front end of the sensor element. Accordingly, in the detection of the air-fuel ratio, it is definitely necessary to heat up the sensitive portion of the air-fuel ratio sensor to its active temperature. Without this warmup operation, the sensor element cannot operate accurately nor its characteristics is stabilized.
For example, Japanese Patent No. 5-46498, published in 1993, discloses this kind of air-fuel ratio sensor. According to this prior art, a heater is united with a fixing metal by soldering and the fixing metal is fixed to the open end of an inside chamber by means of a coil spring.
Furthermore, Japanese Patent No. 6-3430, published in 1994, discloses another air-fuel ratio sensor. According to this prior art, a heater is fixed to an inside chamber by utilizing a fixing metal, although the heater and the fixing metal are not united.
Meanwhile, to satisfy the exhaust gas emission requirements strictly regulated year by year, it is required to detect an air-fuel ratio within a short time period even immediately after an engine is just started up. One measure for solving this requirement is to dispose a heater as close as possible to the sensitive edge of the sensor element. By doing so, heat generated from the heater is effectively used to warm up the sensitive edge of the sensor element. More specifically, it is most preferable to bring the tip end of the heater into contact with the bottom surface of the inside chamber and to keep the contact between the heater and the bottom surface.
However, there are following problems in assembling the heater in the sensor element.
There is a necessity of checking or confirming during the assembling operation that the tip end of the heater is surely brought into contact with the bottom surface of the inside chamber.
Both of the sensor element and the heater are fragile and weak against shock. Hence, if the heater is forcibly inserted into the inside chamber of the sensor element by applying an excessively large force, there will be a possibility that either the sensor element or the heater is damaged.